Digital access arrangement circuitry and method for connecting to phone lines having a DC holding circuit with low distortion and current limiting

ABSTRACT

A digital direct access arrangement (DAA) circuitry may be used to terminate the telephone connections at the user&#39;s end that provides a communication path for signals to and from the phone lines. Briefly described, the DAA provides a programmable means for the DC termination for a variety of international phone standards. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DC holding circuit is provided in which a programmable DC current limiting mode is available. In the current limiting mode, power may be dissipated in devices external to a DAA integrated circuit. Moreover, much of the power may be dissipated in external passive devices, such as resistors. Further, the current limiting mode includes a distortion adjustment circuit to limit distortion at a crossover point at which current limiting effects occur.

This application is a continuation-in-part application of U. S. Ser.Nos. 08/841,409, U.S. Pat. No. 6,137,827, Ser. No. 08/837,702, U.S. Pat.No. 5,870,046, and Ser. No. 08/837,714, U.S. Pat. No. 6,430,229, allfiled on Apr. 22, 1997, and the following U.S. patent applications filedon Mar. 4, 1998: Ser. No. 09/034,687, U.S. Pat. No. 6,359,983, entitled“Digital Isolation System With Data Scrambling” by Andrew W. Krone etal; Ser. No. 09/034,456, U.S. Pat. No. 6,144,326, entitled “DigitalIsolation With ADC Offset Calibration; by Andrew W. Krone et al.; Ser,No. 09/034,455, PENDING, entitled “Ring-Detect Interface Circuitry andMethod for a Communication System” by Timothy J. Dupuis et al.; Ser. No.09/035,779, U.S. Pat. No. 6,389,134, entitled “Call Progress MonitorCircuitry and Method for a Communication System” by Timothy J. Dupuis etal.; Ser. No. 09/034,683, U.S. Pat. No. 6,167,134, entitled “ExternalResistor and Method to Minimize Power Dissipation in DC HoldingCircuitry for a Communication System” by Jeffrey W. Scott et al.; Ser.No. 09/034,620, U.S. Pat. No. 6,160,855, entitled “Caller ID CircuitPowered Through Hookswitch Devices” by Jeffrey W. Scott et al.; and Ser.No. 09/034,682, U.S. Pat. No. 6,408,034, entitled “Framed Delta SigmaData With Unlikely Delta Sigma Data Patterns” by Andrew W. Krone et al.;and Ser. No. 09/035,175, U.S. Pat. No. 6,385,235, entitled “DirectDigital Access Arrangement Circuitry and Method For Connecting To PhoneLines” by Jeffrey W. Scott, Navdeep S. Sooch and David R. Welland, allof which are expressly incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the field of digital access arrangementcircuitry. More particularly, this invention relates to digital accessarrangement circuitry for connecting to a variety of phone linestandards. The digital access arrangement circuitry may further includeisolation barrier utilizing a capacitor coupled isolation barrier.

BACKGROUND

Direct Access Arrangement (DAA) circuitry may be used to terminate thetelephone connections at a phone line user's end to provide acommunication path for signals to and from the phone lines. DAAcircuitry includes the necessary circuitry to terminate the telephoneconnections at the user's end and may include, for example, an isolationbarrier, DC termination circuitry, AC termination circuitry, ringdetection circuitry, and processing circuitry that provides acommunication path for signals to and from the phone lines.

Generally, governmental regulations specify the telephone interfacerequirements and specifications for a variety of parameters including ACtermination, DC termination, ringer impedance, ringer threshold, etc.For example, Federal Communications Commission (FCC) Part 68 governs theinterface requirements for telephones in the United States. However, theinterface requirements world wide are not standardized, and thus, incountries other than the United States the applicable standards mayinclude the TBR21, NET4, JATE, and various country specific PTTspecifications. Because the interface requirements are not standardizedfrom country to country, often different DAA circuitry is required foruse in each country in order to comply with the appropriate standard.The requirement for different DAA circuitry, however, limits the use ofone phone line interface in a variety of countries. Thus, for example, amodem in a laptop computer configured for interfacing with a phone linein one country may not necessarily operate properly in another country.Further, the requirement for different DAA circuitry in variouscountries hinders the design of a single integrated cost effective DAAsolution for use world wide.

As mentioned above, the telephone interface requirements generallyinclude specifications for DC termination of the telephone line. Forexample, the DC impedance that the DAA circuitry presents to thetelephone line (typically ≦300Ω) may be required by regulations to beless than the AC impedance that the DAA circuitry presents to thetelephone line (typically ≈600Ω). Consequently, inductive behavior isrequired from the section of the DAA circuitry that sinks DC loopcurrent, which is typically called the DC termination or DC holdingcircuitry. This inductive behavior of the DC holding circuitry shouldprovide both high impedance and low distortion for voiceband signals.The DC termination specifications may also include limits for themaximum current and power dissipation. For example, the TBR-21specification requires the DC holding circuit to limit DC current toless than 60 mA with a maximum power dissipation of approximately 2watts.

Prior techniques for implementing DC holding circuitry have includedbipolar transistor (e.g., PNP transistor) implementations. These priortechniques, however, have suffered from various disadvantages. Forexample, although bipolar transistor implementations typically present adesired high impedance (e.g., >>600Ω) to the telephone network forvoiceband signals, such implementations are limited. In contrast, a CMOSdesign would be preferable because CMOS technology allows a high levelof integration, for example with other phone line interface functions.CMOS implementations on CMOS integrated circuits, however, may faceconsiderable problems in dissipating the power consumed by the DCholding circuitry. The design of a DC holding circuit for use withmultiple standards may be further complicated in that the variousinternational specifications may conflict with regards to off-hooksettling times and pulse dialing templates (which may require fastsettling time constants) and high speed interface designs (such as foruse in modems) which require very low frequency operation (i.e.approximately as low as 10 Hz). Furthermore, it is desirable toimplement such DC holding circuits in a manner that does not causeexcessive distortion at low and high frequencies.

It is also desirable that the DAA circuitry act as an isolation barriersince an electrical isolation barrier must exist in communicationcircuitry which connects directly to the standard two-wire publicswitched telephone network and that is powered through a standardresidential wall outlet. Specifically, in order to achieve regulatorycompliance with Federal Communications Commission Part 68, which governselectrical connections to the telephone network in order to preventnetwork harm, an isolation barrier capable of withstanding 1000 voltsrms at 60 Hz with no more than 10 milliamps current flow, must existbetween circuitry directly connected to the two wire telephone networkand circuitry directly connected to the residential wall outlet.

There exists a need for reliable, accurate and inexpensive DAA circuitryfor effecting the DC termination characteristics for multiple phone linestandards and a DAA circuitry which also provides the necessaryelectrical isolation barrier.

SUMMARY OF THE INVENTION

The above-referenced problems are addressed by the present invention,which provides a reliable, inexpensive, DAA circuit that may be utilizedwith multiple telephone interface standards and which also provides anisolation system that is substantially immune to noise that affects thetiming and/or amplitude of the signal that is transmitted across theisolating element, thus permitting an input signal to be accuratelyreproduced at the output of the isolation system.

The present invention provides digital direct access arrangement (DAA)circuitry that may be used to terminate the telephone connections at theuser's end to provide a communication path for signals to and from thephone lines. Briefly described, the invention provides a means forproviding DC termination for a variety of international phone standards.The invention may also be utilized with means for transmitting andreceiving a signal across a capacitive isolation barrier. Moreparticularly, a DC holding circuit is provided in which a programmableDC current limiting mode is available. In the current limiting mode,power may be dissipated in devices external to a DAA integrated circuit.Moreover, much of the power may be dissipated in external passivedevices, such as resistors. Further, the current limiting mode includesa distortion adjustment circuit to limit distortion at a crossover pointat which current limiting effects occur.

In one embodiment, a communication system is provided. The system mayinclude phone line side circuitry that may be coupled to phone lines andpowered side circuitry that may be coupled to the phone line sidecircuitry through an isolation barrier. The system may further include aDC holding circuit within the phone line side circuitry, the DC holdingcircuit being programmable in response to data transmitted across theisolation barrier to operate the DC holding circuit in a plurality ofmodes, the DC holding circuit operable in at least a first mode to meeta first phone line interface standard and a second mode to meet a secondphone line interface standard, the second phone line interface standardhaving a DC current limit requirement. The system may also include adistortion adjustment circuit within the DC holding circuit.

In another embodiment, a method of providing a communication system thatmay be coupled to a phone line is provided. The method may comprisecoupling an isolation barrier between powered circuitry and phone lineside circuitry, and forming a DC holding circuit within the phone lineside circuitry, the DC holding circuit comprising a phone line sideintegrated circuit and external circuitry external to the integratedcircuit. The method also includes providing a programmable circuit forswitching the DC holding circuit between at least a first and secondmode of operation, the first mode of operation for at least a firstphone line interface standard and the second mode of operation for atleast a second phone line interface standard, the second standard havinga DC termination current limit. Further, the method may include couplingthe internal circuitry and external circuitry so that if the DC holdingcircuit is operated in the second mode of operation more power may bedissipated in the external circuitry during the second mode of operationthan during the first mode of operation, providing at least twodifferent impedance levels for the DC holding circuit when the DCholding circuit is operated within the second mode of operation, andadjusting distortion effects caused by a transition between the at leasttwo different impedance levels.

In another embodiment, a DC holding circuit for reducing powerdissipation requirements of an integrated circuit within a communicationsystem that may be connected to phone lines is disclosed. The DC holdingcircuit may comprise at least one switchable circuit, the switchablecircuit having a first state for a non-current limiting mode ofoperation and a second state for a current limiting mode of operation,external circuitry external to the integrated circuit, internalcircuitry within the integrated circuit, the external circuitry and theinternal circuitry being coupled together wherein the external circuitrydissipates more power in the current limiting mode than in thenon-current limiting mode, and distortion adjustment circuitry coupledto the internal circuitry, the distortion adjustment circuitry operableat least when the DC holding circuit is in the current limiting mode.

In another embodiment, a method of providing a DC holding circuit isdisclosed. The method may comprise forming the DC holding circuit withinternal circuitry internal to an integrated circuit and externalcircuitry external to the integrated circuit and providing aprogrammable circuit for switching the DC holding circuit between atleast a first and second mode of operation, the first mode of operationfor at least a first phone line interface standard and the second modeof operation for at least a second phone line interface standard, thesecond standard having a DC termination current limit. The method mayfurther include coupling the internal circuitry and external circuitryso that if the DC holding circuit is operated in the second mode ofoperation more power may be dissipated in the external circuitry duringthe first mode of operation than during the second mode of operation,providing at least two different impedance levels for the DC holdingcircuit when the DC holding circuit is operated within the second modeof operation, and adjusting distortion effects caused by a transitionbetween the at least two different impedance levels.

In still another embodiment, a method of forming a DC holding circuitmay include providing integrated circuitry and non-integrated circuitryto comprise the DC holding circuit, the DC holding circuit capable ofmeeting at least at least a first and second phone line interfacestandards, the at least two phone line interface standards havingdiffering current limit specifications, the second standard limiting DCcurrent to a lower amount than the first standard. The method mayfurther include utilizing at least one switchable circuit so that the DCholding circuit may be programmed for at least one of the phone lineinterface standards. Also the method includes coupling the integratedcircuitry and the non-integrated circuitry together so that when the DCholding circuit is programmed for the second phone line interfacestandard, at least one circuit element of the external circuitry willreceive additional DC current as compared to when the DC holding circuitis programmed for the first phone line interface standard. In additionthe method may include providing a plurality of DC impedance levels forthe DC holding circuit when the DC holding circuit is programmed for thesecond phone line interface standard, and limiting distortion at acrossover point between at least two of the plurality of DC impedancelevels.

In another embodiment, Aa DC holding circuit within a communicationsystem that may be connected to phone lines is provided. The DC holdingcircuit may be compatible with a phone line interface standard that hasa DC current limit requirement. The DC holding circuit may comprise afirst impedance level present at one or more inputs of the DC holdingcircuit for a first set of DC loop current values, a second impedancelevel present at the one or more inputs of the DC holding circuit for asecond set of DC loop current values, and distortion adjustmentcircuitry within the DC holding circuit, the distortion adjustmentcircuitry operable to limit distortion during transitions between thefirst and second impedance levels.

In yet another embodiment, a method of providing a DC holding circuitmay include providing at least two different impedance levels for the DCholding circuit when the DC holding circuit is operated in a DC currentlimit mode. The method may further include adjusting distortion effectscaused by a transition between the at least two different impedancelevels.

Also disclosed is method of operating a DC holding circuit, comprisingproviding a plurality of DC impedance levels for the DC holding circuitwhen the DC holding circuit is operated to satisfy a DC current limitstandard, and limiting distortion at a crossover point between at leasttwo of the plurality of DC impedance levels.

DESCRIPTION OF THE DRAWINGS

So that the manner in which the herein described advantages and featuresof the present invention, as well as others which will become apparent,are attained and can be understood in detail, more particulardescription of the invention summarized above may be had by reference tothe embodiments thereof which are illustrated in the appended drawings,which drawings form a part of this specification.

It is noted, however, that the appended drawings illustrate onlyexemplary embodiments of the invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of a telephone set illustrating a typicalapplication of the present invention.

FIG. 2 is a general block diagram of digital DAA circuitry includingphone line side circuitry, an isolation barrier, and powered sidecircuitry according to the present invention.

FIG. 3 is a general block diagram of transmit and receive signal pathswithin digital DAA circuitry according to the present invention.

FIG. 4 is a general circuit diagram of digital DAA circuitry implementedwith two integrated circuits (ICs), a capacitive isolation barrier, andexternal circuitry according to the present invention.

FIGS. 5A-5D are DC termination characteristic curves of various DCtermination modes of the present invention.

FIG. 6 is a general block diagram of a technique for implementingcurrent limiting according to the present invention.

FIG. 7 is a circuit diagram of a DC holding circuit according to thepresent invention.

FIG. 8 is graph of current characteristics of a distortion limitingtechnique according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In order to provide a context for understanding this description, FIG. 1illustrates a typical application for the present invention: a telephonethat includes circuitry powered by a source external to the phonesystem. A basic telephone circuit 118 is powered by the “battery”voltage that is provided by the public telephone system and does nothave a separate power connection. Many modern phones 110, however,include radio (cordless), speakerphone, or answering machine featuresthat require an external source of power 112, typically obtained byplugging the phone (or a power supply transformer/rectifier) into atypical 110-volt residential wall outlet. In order to protect publicphone system 114 (and to comply with governmental regulations), it isnecessary to isolate “powered circuitry” 116 that is externally poweredfrom “isolated circuitry” 118 that is connected to the phone lines, toprevent dangerous or destructive voltage or current levels from enteringthe phone system. (Similar considerations exist in many otherapplications as well, including communication, medical andinstrumentation applications in which this invention may be beneficiallyapplied.) The required isolation is provided by isolation barrier 120.The signal that passes through the isolation barrier 120 is an analogvoice signal in a typical telephone application, but it may also be adigital signal or a multiplexed signal with both analog and digitalcomponents in various applications. In some applications, communicationacross isolation barrier 120 may be unidirectional (in eitherdirection), but in many applications, including telephony, bidirectionalcommunication is required. Bidirectional communication may be providedusing a pair of unidirectional isolator channels, or by forming a singleisolation channel and multiplexing bidirectional signals through thechannel. The primary requirements placed on isolation barrier 120 arethat it effectively prevents harmful levels of electrical power frompassing across it, while accurately passing the desired signal from thepowered side 122 to the isolated side 124, or in the reverse directionif desired.

FIG. 2 is a general block diagram of digital DAA circuitry 110 includingphone line side circuitry 118, an isolation barrier 120, and poweredside circuitry 116 according to the present invention. The isolationbarrier 120 may include one or more capacitors and allow for thetransmission of digital information between the isolation interface 1614in the phone line side circuitry and the isolation interface 1610 in thepowered side circuitry. The phone line side circuitry 118 may beconnected to phone lines of a telephone network system, and the poweredside circuitry 116 may be connected to external controllers, such asdigital signal processors (DSP), that may be part of a communicationdevice, such as a phone or modem.

The powered side circuitry 116, which may be implemented as anintegrated circuit (IC), may communicate with the external controllerthrough a digital interface 1606 and a control interface 1608. Forexample, the digital interface 1606 may have a number of external pinsproviding a serial port interface to the external controller, such as amaster clock input pin (MCLK), a serial port bit clock output (SCLK), aserial port data IN pin (SDI), a serial port data OUT pin (SDO), a framesync output pin (FSYNC_bar) (it is noted that the suffix “_bar” is usedto denote a signal that is typically asserted when at a low logiclevel), and a secondary transfer request input pin (FC). Similarly, thecontrol interface 1608 may have a number of external pins providingcontrol and status information to and from the external controller, suchas a ring detect status pin (RGDT_bar), an off-hook status pin(OFHK_bar), a reset pin (RESET_bar), and multiple mode select pins(MODE). In addition, the digital interface 1606 and the controlinterface 1608 are connected to the isolation interface 1610 so thatcontrol, status, signal and other desired information may be transmittedto and received from the phone line side circuitry 118 across theisolation barrier 120.

The phone line side circuitry 118, which may be implemented as anintegrated circuit (IC), may communicate with the phone lines throughhybrid and DC termination circuitry 1617 (the DC termination circuitryprovides an internal power supply voltage), and determine ring-detectand off-hook status information through off-hook/ring-detect block 1620.In addition, the hybrid and DC termination circuitry 1617 and theoff-hook/ring-detect block 1620 are connected to the isolation interface1614 so that control, status, signal and other desired information maybe transmitted to and received from the powered side circuitry 116across the isolation barrier 120.

In the embodiment depicted, the hybrid portion of the hybrid and DCtermination circuitry 1617 has an output pin QE2 (pin QE2 is alsoutilized for DC termination functions as described below) and an inputpin (RX) that may connect to external telephone interface circuitry suchas hook-switch circuitry and a diode bridge. The hybrid circuitry mayfunction to split the differential signal existing on the phone, whichtypically includes both transmit and receive analog information, into aninternal transmit signal (TX_(INT)) and receive signal (RX_(INT)). It isnoted that the QE2 output pin is used to transmit analog information tothe phone lines, and that the RX pin is labeled to indicate that it isused to receive analog information from the phone lines. These externalpin signals are different than the internal analog transmit signal(TX_(INT)) and analog receive signal (RX_(INT)).

The hybrid and DC termination circuitry 1617 may have a number ofexternal pins that also connect to external telephone interfacecircuitry such as hook-switch circuitry and a diode bridge as shown inFIGS. 2 and 4. For example, the hybrid and DC termination circuitry 1617may have a DC termination pin (DCT), a voltage regulator pin (VREG), twoexternal resistor pins (REXT and REXT2), two filter pins (FILT andFILT2) and a isolated ground pin (IGND). The DC termination circuitryterminates the DC voltage on the phone line and provides an internalpower supply for the phone line side circuitry 118. The DC terminationpin (DCT) receives a portion of the phone line DC current with theremainder flowing through pins QE2 and QB2, depending upon thetermination mode and DC current level. The voltage regulator pin (VREG)allows external regulator circuitry, such as a capacitor, to beconnected to the DC termination circuitry 1617. External resistors and acapacitor may be connected to the two external resistor pins (REXT andREXT2) to set the real and complex AC termination impedancerespectively. The filter pin FILT (along with the capacitor C5) sets thetime constant for the DC termination circuit. The filter pin FILT2 setsthe off hook/on hook transient responses for pulse dialing. The isolatedground pin (IGND) may be connected to the system ground for the poweredside circuitry 116 through a capacitor within the isolation barrier 120and may also be connected to the phone line through a ground connectionwithin external diode bridge circuitry.

The off-hook/ring-detect block 1620 may have external input pinsallowing status information to be provided concerning phone line statusinformation (RNG1, RNG2), such as ring and caller identificationsignals. For example, the first ring detect pin (RNG1) may connect tothe tip (T) lead of the phone line through a capacitor and resistor, andthe second ring detect pin (RNG2) may connect to the ring (R) lead ofthe phone line through a capacitor and resistor. In addition,off-hook/ring-detect block 1620 may have external output pins (QB, QE)that control external off-hook circuitry to enter, for example, anoff-hook state or a limited power mode to get caller identificationinformation. More particularly, the output pins (QB, QE) may beconnected to the base and emitter, respectively, of a bipolar transistorwithin external hook-switch circuitry.

FIG. 3 is a general block diagram of internal transmit (TX) and receive(RX) signal paths within digital DAA circuitry 110 according to thepresent invention. In the embodiment depicted, information maycommunicated in either direction across the isolation barrier 120. It isnoted that FIG. 3 does not depict all of the functional blocks withinpowered side circuitry 116 and phone line side circuitry 118. It is alsonoted that the blocks depicted may be implemented as numerous additionalblocks carrying out similar functions.

In the embodiment of FIG. 3, communications from the phone line sidecircuitry 118 to the powered circuitry 116 are considered receivesignals. Within phone line side circuitry 118, a delta-sigmaanalog-to-digital converter (ADC) 1710 receives an internal analogreceive signal (RX_(INT)), which may be provided for example by hybridcircuitry 1617. The output of delta-sigma ADC 1710 is oversampleddigital data stream in a pulse density modulation format. Thedecoder/encoder circuitry 1708 processes and formats this digitalinformation as desired before sending it across the isolation barrier120 as encoded digital information. For example, decoder/encoder 1708may multiplex control data with the digital stream before it is sentacross the isolation barrier 120. This control data may be any desiredinformation, such as ring detect signals, off-hook detect signals, otherphone line status information or data indicative of the country in whichthe DAA will be utilized (so that the appropriate phone line interfacestandards will be satisfied). Within powered side circuitry 116, thedecoder/encoder 1706 decodes this encoded digital information receivedacross the isolation barrier 120. The digital filter 1702 processes thisdecoded digital stream and converts it into internal digital receivedata (RX_(D)) that may be provided through the digital interface 1606 toan external controller.

Communications from the powered side circuitry 116 to the phone lineside circuitry 118 are considered transmit signals. Within powered sidecircuitry 116, a delta-sigma modulator 1704 receives an internal digitaltransmit signal (TX_(D)), which may be provided for example from anexternal controller through digital interface 1606. The output ofdelta-sigma modulator 1704 is an oversampled digital data stream in apulse density modulation format. The decoder/encoder circuitry 1706processes and formats this digital information as desired before sendingit across the isolation barrier 120 as encoded digital information. Forexample, decoder/encoder 1706 may multiplex control data with thedigital stream. This control data may be any desired information, suchas ring detect signals, off-hook detect signals, or other phone linestatus information. In addition, decoder/encoder 1706 may add framinginformation for synchronization purposes to the digital stream before itis sent across the isolation barrier 120. Still further, decoder/encoder1706 may format the digital data stream so that a clock signal may berecovered within the phone line side circuitry 118. Within phone lineside circuitry 118, the decoder/encoder 1708 may recover a clock signaland may decode the encoded digital information received across theisolation barrier 120 to obtain framing, control or status information.The digital-to-analog converter (DAC) 1712 converts the decoded digitalstream and converts it into internal analog transmit data (TX_(INT))that may be provided as an analog signal through the hybrid circuitry1617 and ultimately to the phone lines.

FIG. 4 is a general circuit diagram of digital DAA circuitry 110implemented with a two integrated circuits (ICs) and a capacitiveisolation barrier 120 according to the present invention. In particular,powered side circuitry 116 may include a powered side integrated circuit(IC) 1802A, and phone line side circuitry 118 may include a phone lineside IC 1802B. External circuitry, such as hook-switch circuitry 1804and diode bridge circuitry 1806, is also shown connected to externalpins of the phone line side IC 1802B. In the embodiment depicted,external pins 1810 of the powered side IC 1802A are connected to anexternal digital signal processor (DSP) and the external pins 1808 areconnected to a external application specific IC (ASIC) or controller.The isolation barrier 120 may include a first capacitor (C1) connectingan external signal (C1A) pin on the powered side IC 1802A to an externalsignal (C1B) pin on the phone line side IC 1802B. In addition, theisolation barrier 120 may have a second capacitor (C2) connecting theisolated ground (IGND) pin on the phone line side IC 1802B to the systemground (GND) pin on the powered side IC 1802A. In addition, the isolatedground (IGND) pin may be connected to node 1812 within diode circuitry1806 (and thereby be connected to the phone line) and the remainingground connections of the external circuitry of the phone line sidecircuitry 118. Typical component values for the various externalcapacitors, resistors, transistors, and diodes for the circuit of FIG. 4are shown in Table 1.

TABLE 1 External Component Values Symbol Value C1 150 pF, 4 kV, X7R,±20% C2, C4 1000 pF, 4 kV, X7R, ±20% C3, C6, C10, C13 0.1 μF, 16 V, ±20%C5 0.1 μF, 50 V, X7R, ±20% C7, C8 680 pF, 300 V, X7R, ±5% C9 22 nF, 300V, X7R, ±20% C11 2200 pF, 50 V, X7R, ±5% C12 0.22 μF, 16 V, ±20% C14 560nF, 16 V, X7R, ±20% C15 0.47 μF, 300 V, ±20% R1, R4, R11, R17, R19, R104.87 KΩ, 1/4 W ±1% R2 400 Ω, 1/10 W ±5% R3 10 Ω, 1/10 W, ±5% R4 2.2 kΩ,1/2 W, ±5% R5, R6 30 kΩ, 1/10 W ±5% R9, R10 30 kΩ, 1/4 W ±5% R13 140 Ω,1/10 W ±5% R14 445 Ω, 1/10 W ±5% R15 18.7 kΩ, 1/4 W ±5% R16 10 kΩ, 1/4 W±5% R18 2.2 KΩ, 1/10 W ±5% Q1 Zetex FMMT 497 Q2 Motorola MMBTA92LT1 Q3Motorola MMBTA42LT1 Q4 Motorola PZT2222AT1 FB1, FB2 Ferrite Bead RV1Sidactor 270 V D1-D4 1N4004 Z1, Z2 Zener Diode 6V

A variety of characteristics of the DAA may be programmable in order toachieve compliance with a variety of regulatory standards. Thus, the DCtermination characteristics, AC termination characteristics, ringerimpedance, or billing tone detector of the DAA circuitry 110 may beprogrammable in order to achieve compliance with a variety of regulatorystandards. For example, the DC current limiting requirements of Frenchand TBR21 standards may be programmable obtained. Further, the lowvoltage requirements of Japan, Italy, Norway, and other countries mayalso be programmable obtained. More particularly, four DC terminationmodes (modes 0, 1, 2, and 3) may be programmed by setting two bits of aprogrammable register through use of the serial port data IN pin (SDI).More particularly, mode 2 is the standard loop voltage mode having nocurrent limiting and with the transmit signal limited to −1 dBm. Thismode is utilized to satisfy FCC and many European country requirements.FIG. 5C illustrates the I-V characteristics of mode 2. As shown in FIGS.5A-5D, the DC voltage across the TIP and Ring lines is plotted as afunction of the DC loop current from the phone line. Within theoperating range of 15 mA to 100 mA, the DC impedance of the DC holdingcircuit is approximately 50Ω (the slope of the I-V curve). The lowvoltage standards required for some countries (for example Norway) willbe met by the low voltage mode 0 shown in FIG. 5A with the transmitsignal limited to −5.22 dBm. The slightly higher (approximately 0.3Vhigher) low voltage requirements of other countries (for example Japanand Italy) may be met by the low voltage mode 1 shown in FIG. 5B withthe transmit level limited to −2.71 dBm. As with mode 2, both lowvoltage modes 0 and 1 operate with a DC impedance of the DC holdingcircuit at approximately 50Ω. FIG. 5D illustrates the I-Vcharacteristics of mode 3 which is a current limiting mode as requiredin France and under the TBR21 standard. As shown in FIG. 5D, a firstsegment A of the I-V curve operates at a 50Ω impedance and a secondsegment B of the I-V curve operates at a 3200Ω impedance so that the DCtermination will current limit before reaching 60 mA (i.e. less than 60mA at approximately 35 volts or less. The crossover point between thetwo portions A and B of the curve is indicated as point C. A thirdsegment D of the I-V curve of FIG. 5D operates at an 800Ω impedance.

The data for the particular country the DAA will be utilized within (andthus the required telephone line interface standards) may be transmittedacross the capacitive barrier 120 with the various other DAA controlsignals. The phone line side circuitry 118 can then be programmablyconfigured to satisfy the different various international DC terminationrequirements. Thus, a digitally programmable system is provided in whichcontrol bits can be provided across the isolation barrier to program thephone line side circuitry 118 in a manner such that a wide variety ofphone line interface standards can be satisfied. Moreover, theprogrammable nature of the phone line side circuitry 118 may minimizethe need for changing the external components utilized for coupling thephone line side circuitry 118 to the phone line TIP and RING lines. Inthis manner a single DAA system may be utilized in a cost effectivesoftware programmably manner for world wide use.

In order to programmable achieve the DC termination characteristics ofFIGS. 5A-5C, the DC termination or DC holding circuit of the presentinvention provides a variety of improvements over the prior art. Forexample in order to achieve the current limiting requirements (such asin the TBR21 standard), the phone line side circuitry 118 must dissipateup to approximately two watts of power. Typical non current limitingspecifications such as FCC standards will result in only a fraction ofthat amount of power dissipation to occur. However, it is undesirable torequire this increase power dissipation to be performed by the phoneline side integrated circuit 1802B. The circuit of FIG. 6 provides amechanism in which the increased power dissipation requirements ofcurrent limiting standards may be achieved by dissipating the additionalpower external to the integrated circuit. In this manner, a single DAAsystem may be utilized for both current limiting DC terminationstandards and non-current limiting standards without requiring excessivepower dissipation within an integrated circuit.

As shown in FIG. 6, a phone line side integrated circuit 1802B includesDC termination or DC holding circuitry 600 which is coupled to the DCT,QE2 and QB2 pins. The DCT pin is coupled to a resistor RA, for example a1600Ω resistor. The QB2 pin is coupled to a resistor RB, for example a1600Ω resistor. Though shown as single resistors, each resistor RA andRB may be formed from a plurality of resistors such as resistors R1,R11, and R17, and R4, R19, and R20 respectively as shown in FIG. 4.Resistors RA and RB are coupled to the hookswitch circuitry such asshown in FIG. 6. The QE2 and QB2 pins are coupled to the emitter andbase of transistor Q4 respectively. In operation, the DC current on fromthe phone line may be directed through resistors RA and RB in varyingamounts through control of transistor Q4 in order to adjust the DCimpedance seen by the phone lines. For example, the 50Ω impedancesection of the I-V curve of FIG. 5D (segment A) may be obtained when thetransistor Q4 is fully on and the bulk of the DC current passes throughtransistor Q4. The 3200Ω impedance section of the I-V curve of FIG. 5D(segment B) may be obtained while the transistor Q4 is being turned offand thus actively steering current through resistors RA and RB. The 800Ωimpedance section of the I-V curve of FIG. 5D (segment D) may beobtained when the transistor Q4 is fully turned off and thus the DCcurrent is split between the resistors RA and RB.

The DC termination mode may be selectably programmed through the poweredside circuitry 116 and control information transmitted across thecapacitive barrier 120 to the DC holding circuitry 600. Moreparticularly, the DC holding circuitry controls transistor Q4 dependingupon the selected mode. Thus when current limiting is desired,additional current may be steered to the resistors RA and RB. In thismanner the higher impedance needed for current limiting specificationssuch as the 3200Ω impedance section of the I-V curve of FIG. 5D may beaccurately achieved. Further, the additional power dissipation isperformed external to the phone line side integrated circuit 1802B byresistor RA, resistor RB, and transistor Q4. Thus in one exemplary,resistor RA and resistor RB may each dissipate up to approximatelythree-fourths of a Watt, transistor Q4 up to one-half Watt while theintegrated circuit need only dissipate up to three-tenths of a Watt.This technique is particularly advantageous in that much of the power isdissipated in passive elements (resistors) rather than solely in activedevices. Thus, more than 50% of the DC power dissipated by the DCholding circuit may be dissipated in devices external to the integratedcircuit 1802B, and more particularly, more than 50% of the DC power maybe dissipated in passive resistor devices.

A DC holding circuit 700 for implementing the DC terminationcharacteristics discussed above is shown in FIG. 7. FIG. 7 illustratesportions of the DAA system with like reference numbers and letters asshown in FIG. 4. As can be seen, FIG. 7 includes circuitry both internaland external to the phone side integrated circuit 1802B. Moreparticularly, FIG. 7 includes the RX, DCT, QB2, QE2, and FILT pins andassociated internal and external circuits (the hookswitch circuitry isnot being shown). As shown in FIG. 7, the DC holding circuit 700includes switches S1, S2, S3, S4, S5, S6, and S7. As discussed in detailbelow, the switches may be utilized to select the current limiting ornon-current limiting modes of operation, to switchably operate the DCholding circuit in order to achieve fast settling times and lowfrequency operation and to select the low voltage modes of operation.

The DC holding circuit 700 also includes a current limiting circuitblock 705, a distortion adjustment circuit block 710, and a voltageselection circuit block 715. As discussed further below, the currentlimiting circuit block 705 operates in conjunction with proper selectionof switches to implement the higher effective impedance of the DCholding circuit to achieve the desired current limiting effect at aselected current limiting crossover point. The external transistor Q4 iscontrolled so that in the current limiting mode of operation current maybe steered to both resistors RB (which as described above may each beformed from multiple resistors) so that power. may be dissipatedexternal to the integrated circuit 1802B. The distortion adjustmentcircuit block 710 operates to lower the total harmonic distortion at thecrossover point. The voltage selection circuit block 715 is utilized toselect either of the low voltage modes (modes 0 and 1) or the standardvoltage mode (mode 2 or 3) The remaining portions of the DC holdingcircuit 700 operate in both current limiting and non-current limitingmodes as a second order (two pole) system with external capacitors C12and C5 affecting the frequency of the poles. The components of the DCholding circuit may be configured in a wide variety of manners to obtainthe advantages of the invention disclosed herein and the embodiment ofFIG. 7 is merely exemplary. Likewise a wide variety of component valuesmay be utilized. In one embodiment, the component values may be selectedas shown below in Table 2. The transistors may be sized as labeled “_X”in FIG. 7.

TABLE 2 Internal Component Values Symbol Value R101, R102 98 kΩ, R103 60kΩ, R104 30 kΩ, R105 50 kΩ, R108 10 kΩ, R109 2 MΩ, R110 2 kΩ, R111, R11250 kΩ, R113 12.5 kΩ, R114 4.16 kΩ, R115 2.32 kΩ, I1 430 μA I2 60 μA

When the DC holding circuit 700 of FIG. 7 is operating in thenon-current limiting mode (modes 0, 1, or 2), switch S3 is open. Duringthe current limiting mode of operation (mode 3) switch S3 is closed. Aswill be described in more detail below, switches S1, S2, and S4 operateto selectably control time constants of the DC holding circuit 700 foruse with PTT specifications which may conflict with very low frequencyoperations. Switches S5 and S6 are utilized to select the low voltagemodes of operation (modes 0 and 1). More particularly, in standardvoltage level operation (modes 2 and 3) both switches S5 and S6 areclosed. In low voltage mode 0, switches S5 and S6 are both opened. Inlow voltage mode 1, switch S5 is open and switch S6 is closed. Inoperation, the selection of the state of the switches S5 and S6 willvary the resistance seen at the negative input of op amp OA2, thuschanging the DC voltage at the DCT pin, which in turn changes thevoltage between the TIP and RING lines for a given amount of DC loopcurrent. In mode 0 the DC voltage at the DCT pin is 2.8V, in mode 1 3.1Vand in modes 2 and 3 4.0V.

Current Limiting

As mentioned above, in the current limiting mode of operation (mode 3)switch S3 is closed and in the non-current limiting modes (modes 0-3)switch S3 is opened. The operation of the current limiting mode isdiscussed below with the time constant control switches set to a operatein S1 open, S2 closed and S4 open (time constant phase 1) forillustrative purposes. However, the current limiting mode may also beoperated with the time constant phase 2 (S1 closed, S2 open and S4closed) selected.

During non-current limiting modes of operation (transistor Q4 is fullyturned), the DC impedance of the DC holding circuit 700 of FIG. 7 isapproximately 50Ω when utilizing the illustrative component values ofTables 1 and 2. This impedance value is obtained as explained below. Theop amp circuitry of OA1 and OA2 attempt to force the DCT pin to trackthe AC signal on the TIP and RING lines with the resistor ratiosselected in the illustrative embodiment. The op amp circuitry alsoattempts to prevent the AC current component in the current throughtransistor M1 (and transistor M3 which is tied to the gate of transistorM1). The resulting DC current through transistor M1, I(M1), will thus beproportional to the DC line voltage sinceI(M1)=(V_(line)(DC)−V_(hookswitch)(DC)−V_(diode bridge)(DC)−V_(DCT)(DC))/RA,where V_(hookswitch)(DC) is the DC drop across the hookswitch circuitry,V_(diode bridge)(DC) is the DC drop across the diode bridge circuitryand V_(DCT) is the DC voltage at the DCT pin.

Further, the DC current at pin QE2 (and thus transistor Q4), will be afunction of the current mirror transistors M6 and M7. More particularlywith the 1X:63X sizing of the current mirror transistors shown in FIG.7, the DC current at pin QE2, I(QE2), will be approximately 64×I(M3).Since I(M3)=I(M1)/2, I(QE2)=32×I(M1). Further since I(M1)=V_(line)/RA+k,I(QE2)=V_(line)/(RA/32)+k, wherein k is a constant. Thus, with RA chosento be 1600Ω the desired DC termination impedance of 50Ω will result inthe non-current limiting mode of operation.

When the current limiting mode of operation is entered, the switch S3will be closed. This will allow current to sink through resistor R108and transistor M10. Thus, the gate voltage on transistors M1 and M3 willnot necessarily be the same. More particularly, when switch S3 is closedthe current limiting effect will begin to occur as a function of thevalue of the DC current source I1 since the current limiting circuitblock 705 will attempt to maintain I1>I(M2)+I(M4). When the loop currentis low, and thus the gate voltages on transistors M2 and M4 is at alevel such that I1>I(M2)+I(M4), current is not sunk through transistorM10 and the current limiting block 705 does not have an effect. At thispoint the circuit will be operating in the region of segment A of themode 3 operation shown in FIG. 5D.

However, as the DC loop current increases, the current throughtransistors M2 and M4 will increase. When the total current I(M2)+I(M4)reaches the value of I1, the current limiting effect will begin byreducing I(M4) as I(M2) increases by sinking current through resistorR108 and transistor M10. In this manner the relationship I1=I(M2)+I(M4)may be maintained. This has the effect of reducing the current throughtransistor M3 and thus actively steering current out of the QE2 pin andinto resistors RA and RB. At this point the circuit will be operating inthe region of segment B of the mode 3 operation shown in FIG. 5D. Thelocation of the crossover point C (the point of change of DC impedance)of FIG. 5D is thus dependent upon the value of I1. In the illustrativeembodiment shown, I1 may be 430 μA to achieve a current limitingcrossover point at approximately 45 mA of DC loop current.

Distortion Limiting at Current Crossover

The current limiting technique discussed above has potential to increasethe harmonic distortion at the crossover (or “knee”) point of the DC I-Vcurve of FIG. 5D. More particularly, though ideally the current throughtransistor M3 has no AC component, in practice non-ideal circuitcomponents, mismatches, etc. will result in some AC component of thecurrent through M3. Thus, the total phone line current, i_(LINE), willinclude the DC loop current of the holding circuit, the AC phone signal,and AC component of the current in M3. Distortion in the AC component ofthe current in M3 will therefore add harmonic distortion to the phoneline signal. The current limiting techniques discussed above will adddistortion to the AC component of the M3 current when the DC loopcurrent is located at the crossover point or close to it. Moreparticularly, in such situations the AC component of the transistor M3current will have result of repeatedly turning on and off the currentlimiting effect. This will result in the AC component of the current intransistor M3 to also be repeatedly limited or not limited, thusdistorting the AC component. For example, when the DC loop current islocated at the crossover point and a low frequency sine wave is appliedon the phone line, the AC component of the current through transistor M3may be clipped as shown by curve A of FIG. 8. As shown in FIG. 8, theclipping of curve A will occur when the total current through M3 exceedsthe value of the current limit level, I1. This distortion will be mostsignificant for relatively low frequency signals (about less than 100Hz) due to the low pass filtering effect of capacitor C5 on the outputof OA2 which tends to remove higher frequency components (note that C5provides a high pass filter to node RX which in turn performs a low passfiltering effect on the gate of transistor M1 through the use of OA1 andOA2).

The distortion adjustment circuit block 710 of FIG. 7 compensates forthis clipping effect through control of transistor M14 which is alsocoupled to the QE2 pin. The distortion adjustment circuit block 710operates in the mode 3 current limiting mode through the closure ofswitch S7. In other modes, switch S7 is opened and the distortionadjustment circuit block 710 does not affect the DC holding circuitry.The distortion adjustment circuit block 710 operates such that thecurrent through transistor M14 has a response opposite to that of thecurrent through transistor M3 such as shown by curve B of FIG. 8.Because both transistors M3 and M14 are coupled to the QE2 pin, thetotal AC component effect of the current through transistors M3 and M14will sum together. Since curves A and B FIG. 8 demonstrate oppositeclipping effects, the summation of these currents will be relativelyfree of clipping and the associated distortion, at least to a firstorder. The current response of curve B is obtained through the currentsteering relationship of transistors M3, M12, M11 and M14. Thus, withthe transistor sizing shown in FIG. 7, the relationshipi(M14)=(10×I2)−i(M3) will result and the resulting AC component seen bythe phone line due to the AC component of the current in transistors M3and M4 will be 1033 I2. The value of I2 may be chosen such that I2 isgreater than I(M3)/10 at the crossover point.

2^(nd) Order DC Holding Circuit

The DC holding circuit 700 of FIG. 7 is further advantageous in that itis a second order DC holding circuit. More particularly, a first andsecond pole in the frequency response of the circuit is provided throughthe use of capacitors C5 and C12 respectively. The first pole resultsfrom the filtering action at the RX pin resulting from capacitor C5 andthe associated resistors coupled to the RX pin. This filtering action isrelatively sufficient at high frequencies (for example 100 Hz orgreater) to result in very little AC signals on the common gate line oftransistors M1 and M3 (and thus low AC current components through thosetransistors). However, at low frequencies more AC current componentswill be present in transistors M1 and M3 which would result indistortion at low frequencies. Improved frequency response may beobtained by adding a second frequency pole to the system. For example,another stage of low pass filtering could be added between the gates oftransistor M1 and M3 to more heavily filter the gate signal ontransistor M3. Alternatively, as shown in FIG. 7 the additional low passfiltering may be provided through the use capacitor C12 coupled to theQE2 pin. The use of the filter capacitor C12 coupled to the QE2 pin alsoprovides noise filtering of the large PMOS device M7 which is used as alarge current sinking device.

Thus, a second order DC holding circuit is provided. The use of a secondorder frequency response circuit provides a DC holding circuit which mayhave greater than 60 dB THD at 100 Hz, 20 mA, −1 dBm. The second orderDC holding circuit is shown in one implementation to have two filtercapacitors (C5 and C12) placed external to the phone side integratedcircuit 1802B, however, other circuit techniques may be utilized toachieve a second order DC holding circuit. It is desirable that thefrequency poles by low frequency poles, such as at or below 300 Hz, andmore particularly below 50 Hz. In the illustrative implementation, thefirst filter resulting from capacitor C5 provides a first pole at 16 Hz(a low pass filter effect on the gate of transistor M1). The secondfilter resulting from capacitor C12 provides a second pole at 0.44 Hz.

Switchable Time Constants

It is generally desirable that the DC holding circuit present aimpedance at DC and at AC frequencies the DC circuitry is removed fromthe signal path. One way to achieve such performance would be to providea DC circuit which operates very slowly such that it is cut off atfrequencies above several hertz. This may be particularly important whentransmitting very low frequency modem signals (down to approximately 10hertz) which have low distortion requirements such as greater than 75 dBTHD for frequencies greater than 300 Hz (full scale), 60 dB THD forfrequencies greater than 100 Hz (full scale), and), and 80 dB THD forfrequencies greater than 100 Hz (at −9 dBm) However, the use of veryslow DC holding circuitry would conflict with phone line interfacestandards in many PTT specifications. For example, some interfacestandards which require rapid on-hook and off-hook switching. Forexample, the settling time for switching between on-hook and off-hookconditions may be required to be greater than 90% loop current settlingin 20 msec from an off hook event. Such time constraints may beparticularly important for pulse dialing.

The present invention may include the use of switchable time constantswhich affect the speed of the DC holding circuitry. Thus, the DC holdingcircuitry may be operated in a first phase (phase 1) which has a fastsettling time and in a second phase (phase 2) which has slow settlingtime to allow low frequency operation. Thus, the DC holding circuit maybe utilized to meet the standards for rapid on/off-hook operation (suchas in pulse dialing) and then after the phone line goes off-hook the DCholding circuit may be switched to slower circuit operation to allow lowfrequency phone line signal operation. In this manner a DC holdingcircuit having a variable operating frequency is provided.

The phase of operation, high speed phase 1 or low speed phase 2, iscontrolled by switches S1, S2, and S4. During high speed phase 1, switchS1 is closed, S2 opened and S4 closed. Closing switch S1 and openingswitch S2 results in removing the first frequency pole (caused bycapacitor C5) from the DC holding circuit. Further, closing switch 4increasing the second frequency pole to 360 Hz since the time constantof the loop current settling is now set by capacitor C12 and resistor110 in parallel with resistor R109. The value of the capacitor andresistors may be selected (as shown above) to provide proper settlingwithin a few milliseconds to give fast pulse dialing settling. It isnoted that during phase 1, capacitors C5 and C12 will charge to theirappropriate values. This charging will help minimize transient glitcheswhen switching from phase 1 to phase 2. When the DC holding circuit isswitched to phase 2, the standard second order DC holding circuitoperation described above results. Phase 2 may be set to activate atapproximately 200 msec after off-hook conditions occur. The switchingbetween phase 1 and phase 2 conditions may be utilized with,all of themodes of operation (modes 0-3) described above.

Thus, a DAA DC holding circuit operable in two phases is provided. Thefirst phase may be a fast mode of operation used during the transmissionof signaling information such as establishment of off-hook conditions orpulse dialing. The second phase may be a slow mode of operation used forthe transmission of phone user data (such as, for example, voice data ormodem data). The DC holding circuit may be in the first phase until sometime period after off-hook conditions are last detected (for example 200msec). Thereafter the DC holding circuit may be switched to the secondphase. The time constant of the circuit for establishing off-hookconditions (the first phase) may be relatively fast or short, typicallyless than 10 msec, more preferably less than 5 msec and in theillustrative embodiment less than 1 msec. The time constant of thecircuit during user data transmission (the second phase) may berelatively slow or long, typically greater than 100 msec, morepreferably greater than 200 msec and in the illustrative embodimentapproximately 400 msec.

Further modifications and alternative embodiments of this invention willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the invention. It is to be understood that the forms ofthe invention herein shown and described are to be taken as thepresently preferred embodiments. Various changes may be made in theshape, size and arrangement of parts. For example, equivalent elementsmay be substituted for those illustrated and described herein, andcertain features of the invention may be utilized independently of theuse of other features, all as would be apparent to one skilled in theart after having the benefit of this description of the invention.Moreover, the various aspects of the inventions disclosed herein may beused in combination or separately as will also be apparent to thoseskilled in the art. For example, though the current steering is shownherein with regard to a programmable DAA, the use of external devices todissipate substantial amounts of power for DC current limited standardsmay be utilized in a non-programmable DAA.

We claim:
 1. A communication system, comprising: phone line side circuitry capable of being coupled to phone lines; powered side circuitry capable of being coupled to the phone line side circuitry through an isolation barrier; a DC holding circuit within the phone line side circuitry, the DC holding circuit being programmable in response to data transmitted across the isolation barrier to operate the DC holding circuit in a plurality of modes, the DC holding circuit operable in at least a first mode to meet a first phone line interface standard and a second mode to meet a second phone line interface standard, the second phone line interface standard having a DC current limit requirement; and a distortion adjustment circuit within the DC holding circuit.
 2. The communication system of claim 1, the DC holding circuit comprising a phone line side integrated circuit and at least one external device, the external device dissipating more power in the second mode than in the first mode.
 3. The communication system of claim 1, further comprising the isolation barrier coupled between the phone line side circuitry and the powered side circuitry, the isolation barrier being a capacitive barrier.
 4. The communication system of claim 1, wherein the phone line side circuitry and the powered side circuitry are configured to communicate across the isolation barrier through digital signals.
 5. The communication system of claim 4, further comprising the isolation barrier coupled between the phone line side circuitry and the powered side circuitry, the isolation barrier comprising one or more capacitors.
 6. The communication system of claim 1, wherein the DC holding circuit has at least two different impedances when operating in the second mode, the distortion adjustment circuit operable to limit distortion at a transition between the two impedances when the DC holding circuit is in the second mode.
 7. The communication system of claim 6 wherein a substantial portion of the power dissipated by the DC holding circuit in the second mode is dissipated external to the phone line side integrated circuit.
 8. The communication system of claim 7, wherein 50% or more of the power dissipated by the DC holding circuit is dissipated external to the phone line side integrated circuit.
 9. The communication system of claim 8, wherein 50% or more of the power dissipated by the DC holding circuit is dissipated in one or more passive external devices.
 10. The communication system of claim 9, wherein the one or more passive external devices are one or more resistors.
 11. A method of providing a communication system capable of being coupled to a phone line, comprising: coupling an isolation barrier between powered circuitry and phone line side circuitry; forming a DC holding circuit within the phone line side circuitry, the DC holding circuit comprising a phone line side integrated circuit and external circuitry external to the integrated circuit; providing a programmable circuit for switching the DC holding circuit between at least a first and second mode of operation, the first mode of operation for at least a first phone line interface standard and the second mode of operation for at least a second phone line interface standard, the second standard having a DC termination current limit; coupling the internal circuitry and external circuitry so that if the DC holding circuit is operated in the second mode of operation more power may be dissipated in the external circuitry during the second mode of operation than during the first mode of operation; providing at least two different impedance levels for the DC holding circuit when the DC holding circuit is operated within the second mode of operation; and adjusting distortion effects caused by a transition between the at least two different impedance levels.
 12. The method of claim 11, further comprising utilizing a capacitive barrier to isolate the powered circuitry and the phone line circuitry.
 13. The method of claim 11, further comprising passing digital data across the isolation barrier.
 14. The method of claim 13, further comprising utilizing a capacitive barrier to isolate the powered circuitry and the phone line circuitry.
 15. The method of claim 11, the adjusting distortion effects further comprising compensating for the effects of AC signals within the DC holding circuit.
 16. The method of claim 15, the adjusting distortion effects further compensating for the clipping of AC signals.
 17. A DC holding circuit for reducing power dissipation requirements of an integrated circuit within a communication system capable of being connected to phone lines, the DC holding circuit comprising: at least one switchable circuit, the switchable circuit having a first state for a non-current limiting mode of operation and a second state for a current limiting mode of operation; external circuitry external to the integrated circuit; internal circuitry within the integrated circuit, the external circuitry and the internal circuitry being coupled together wherein the external circuitry dissipates more power in the current limiting mode than in the non-current limiting mode; and distortion adjustment circuitry coupled to the internal circuitry, the distortion adjustment circuitry operable at least when the DC holding circuit is in the current limiting mode.
 18. The DC holding circuit of claim 17, the DC holding circuit having at least two impedance levels when the DC holding circuit is in the current limiting mode of operation.
 19. The DC holding circuit of claim 18, the distortion adjustment circuitry limiting distortion occurring when the DC holding circuit is operating close to a transition point between two of the at least two impedance levels.
 20. A method of providing a DC holding circuit, comprising: forming the DC holding circuit with internal circuitry internal to an integrated circuit and external circuitry external to the integrated circuit; providing a programmable circuit for switching the DC holding circuit between at least a first and second mode of operation, the first mode of operation for at least a first phone line interface standard and the second mode of operation for at least a second phone line interface standard, the second standard having a DC termination current limit; coupling the internal circuitry and external circuitry so that if the DC holding circuit is operated in the second mode of operation more power is capable of being dissipated in the external circuitry during the first mode of operation than during the second mode of operation; providing at least two different impedance levels for the DC holding circuit when the DC holding circuit is operated within the second mode of operation; and adjusting distortion effects caused by a transition between the at least two different impedance levels.
 21. The method of claim 20, the adjusting distortion effects further comprising compensating for the effects of AC signals within the DC holding circuit.
 22. The method of claim 21, the adjusting distortion effects further compensating for the clipping of AC signals.
 23. The method of claim 22, the compensation of clipping of AC signals comprising providing an additional signal having substantially the opposite waveform of a clipped AC signal.
 24. A method of forming a DC holding circuit, comprising: providing integrated circuitry and non-integrated circuitry to comprise the DC holding circuit, the DC holding circuit capable of meeting at least at least a first and second phone line interface standards, the at least two phone line interface standards having differing current limit specifications, the second standard limiting DC current to a lower amount than the first standard; utilizing at least one switchable circuit so that the DC holding circuit is capable of being programmed for at least one of the phone line interface standards; coupling the integrated circuitry and the non-integrated circuitry together so that when the DC holding circuit is programmed for the second phone line interface standard, at least one circuit element of the external circuitry will receive additional DC current as compared to when the DC holding circuit is programmed for the first phone line interface standard; providing a plurality of DC impedance levels for the DC holding circuit when the DC holding circuit is programmed for the second phone line interface standard; and limiting distortion at a crossover point between at least two of the plurality of DC impedance levels.
 25. The method of claim 24, the limiting of distortion further comprising compensating for the effects of AC signals within the DC holding circuit.
 26. The method of claim 25, the limiting of distortion further compensating for the clipping of AC signals.
 27. A DC holding circuit within a communication system capable of being connected to phone lines, the DC holding circuit compatible with a phone line interface standard that has a DC current limit requirement, the DC holding circuit comprising: a first impedance level present at one or more inputs of the DC holding circuit for a first set of DC loop current values; a second impedance level present at the one or more inputs of the DC holding circuit for a second set of DC loop current values; and distortion adjustment circuitry within the DC holding circuit, the distortion adjustment circuitry operable to limit distortion during transitions between the first and second impedance levels.
 28. The DC holding circuit of claim 27, the distortion adjustment circuitry compensating for clipping of AC signals.
 29. A method of providing a DC holding circuit, comprising: providing at least two different impedance levels for the DC holding circuit when the DC holding circuit is operated in a DC current limit mode; and adjusting distortion effects caused by a transition between the at least two different impedance levels.
 30. The method of claim 29, the adjusting distortion effects further comprising compensating for the effects of AC signals within the DC holding circuit.
 31. The method of claim 30, the adjusting distortion effects further compensating for the clipping of AC signals.
 32. The method of claim 31, the compensation of clipping of AC signals comprising providing an additional signal having substantially the opposite waveform of a clipped AC signal.
 33. A method of operating a DC holding circuit, comprising: providing a plurality of DC impedance levels for the DC holding circuit when the DC holding circuit is operated to satisfy a DC current limit standard; and limiting distortion at a crossover point between at least two of the plurality of DC impedance levels.
 34. The method of claim 33, the limiting of distortion further comprising compensating for the effects of AC signals within the DC holding circuit.
 35. The method of claim 34, the limiting of distortion further compensating for the clipping of AC signals. 